Energy absorbers which employ elastic or plastic deformation of certain materials to absorb energy are well known. U.S. Pat. Nos. 4,117,637, 4,499,694 and 4,593,502 describe forms of cyclic shear energy absorbers which in seismic isolation applications are typically interposed between two vertically contiguous weight holding members of a structure, such as between the piles and a base floor of a medium size building or between the foundation blocks and upright supports in a bridge. Typically around fifty to one hundred such energy absorbers are used in a building in the form of bearings in the building foundations to seismically isolate the building. A general discussion of these and related devices is given in "An Introduction to Seismic Isolation", R. I. Skinner, W. H. Robinson and G. H. McVerry, Wiley, 1993.
Such bearings used for seismic isolation for example typically comprise a resilient support pad which takes a portion of the weight of the structure and an energy absorbing core comprising a material which deforms plastically as the structure sways relative to the ground. The support pad typically comprises alternating layers of an elastic material such as rubber and plates of a relatively stiff material such as steel bonded together. The energy absorbing core is typically formed of lead. The bearing may be fixed between connector plates which are in turn connected to the building structure members above and below. Alternatively the bearing may simply be fitted into a recess or equivalent on each building structure member. Such energy absorbers are commonly known as lead-rubber bearings.
Lead is the preferred material for forming the energy absorbing core for several reasons. First it yields at a room temperature shear stress of around 10.5 MPa which is low compared with other metals and equivalent plastic materials. Second it restores its mechanical properties through recrystallisation and associated processes relatively rapidly following yield deformations, which provides outstanding resistance to work hardening under cyclic shear at ordinary temperatures. Third lead is readily available at the purity required to exhibit these properties.